HTC

Android devices made by HTC have a few special startup modes that aren't really documented very well, so I'll write them up here in the hope someone else might get some use out of them.

In all these cases, start with your device powered off completely. Pull the battery to make sure it's off if neccesary.

Press and hold Back, power on:

Start in Bootloader. Press Volume Down to start HBoot. Not very useful, but the boot screen tells you various version numbers.

Press and hold Home + Back, power on:

Clear Data. Doing this resets your device to factory defaults. Useful if something got messed up at OS or application level.

Press and hold Home, power on:

Recovery mode. On my Hero, this shows a danger triangle with an exclamation mark. In this mode, you can connect your device to a PC with the USB cable and then run HTC's ROM Update Utility (RUU) to (re)flash your device's ROM. Depending on your device, you might need to press Home + End Call to start the process. Useful when your device no longer boots.

Have any additions, corrections or reports on devices where these work/don't work? Let me know in the comments!